1. Field of the Invention
The present invention relates to a driving circuit for a semiconductor laser diode and an optical transmitter using the driver circuit.
2. Related Prior Art
A driver circuit for the semiconductor laser diode (hereinafter denoted as LD) has been well known, in which a current supplied to the LD is controlled so as to stabilize the output power and the extinction ratio based on the photocurrent generated by the monitoring photodiode (herein after denoted as PD). For instance, a Japanese patent published as H04-139779A has disclosed an LD driver with the AC-coupled configuration, in which the driver controls the bias current supplied to the LD based on the average photocurrent and controls the modulation degree based on the average and the peak value of the photocurrent.
The emission characteristic of the LD and the optical coupling efficiency with the monitoring PD are different from individual LDs. Accordingly, the LD driver is preferable to vary the conversion efficiency of the photocurrent into a corresponding voltage signal depending on the emission characteristic and the coupling efficiency with the PD.
However, a trans-impedance-amplifier (hereinafter denoted as TIA), which is widely used for the current-to-voltage converter, inherently forms a low-pass-filter (LPF) by the resistance of the trans-impedance and a junction capacitance of the PD. Accordingly, to vary the trans-impedance to change the current-to-voltage conversion efficiency results in the degradation of the high cutoff frequency of the TIA, which causes an error in the detection of the peak and bottom values of the output from the TIA.
Therefore, the present invention is to provide an LD driver that may vary the conversion ratio from the current to the voltage with reduced variation in the monitoring efficiency caused by the change of the conversion ratio.